1. Field of the Invention
The present invention relates to a vibration wave driving apparatus equipped with a vibration member comprised of an elastic member to which an electro-mechanical energy conversion element is fixed and a contact member kept in contact with a surface of the vibration member; and configured to generate vibration in the surface of the vibration member by the electro-mechanical energy conversion element to move the contact member as a movable member relative to the vibration member.
2. Description of Related Art
The vibration wave driving apparatus, such as a vibration wave motor (ultrasonic motor) and the like, has a vibration member, which forms vibration in an elastic member of metal or the like to which an electro-mechanical energy conversion element is fixed, when alternating signals of alternating voltages or the like are supplied to the electro-mechanical energy conversion element such as a piezoelectric element or the like, and a movable member (contact member) kept in contact with the vibration member while being pressed against it; and is configured to move the movable member relative to the vibration member by the vibration formed in the vibration member. The vibration wave motors with the vibration member being fixed and the contact member being the movable member are actuators from which a large driving force can be withdrawn at low speed, and have the feature of minimal speed unevenness.
Particularly, smoother driving can be implemented by the vibration wave motors in which vibration of a travelling wave is generated in the elastic member and in which the movable member in contact with the elastic member is driven.
FIG. 15 shows a configuration of a conventional vibration wave motor. This vibration wave motor is composed of a ring-shaped elastic member 1 of metal or the like fixed to a housing 7 with screws; a movable member 4 kept in frictional engagement with the elastic member 1 through a frictional member 3; and a press spring 5 and a rotational shaft 6 for keeping the movable member 4 in press contact with the elastic member 1 and for outputting rotation of the movable member 4, respectively. The rotational shaft 6 is rotatably supported on the housing 7 by ball bearings 8. The elastic member 1 is integrally constructed of a ring-shaped vibration part 1A located in the outermost peripheral region, a support circular plate part 1B located inside of the vibration part 1A, and a flange part 1C located further inside of the support circular plate part 1B. The elastic member is made by cutting or by die forming, such as powder sintering or the like, of a metal material. The vibration member is formed by bonding a piezoelectric element to one surface of the elastic member 1 with an adhesive or the like.
FIG. 16 is a perspective view of the vibration member of the conventional vibration wave motor. A plurality of radial grooves 4 are formed in the axial direction in one plane of the ring-shaped vibration part 1A, and a composite resin material containing PTFE as a principal component is bonded as the frictional member 3 onto upper surfaces of projections 1E of comb teeth shape formed by the plurality of grooves. The frictional member 3 can be a surface-treated metal material or a ceramic material of alumina, depending upon applications.
A ring-shaped piezoelectric element 2 as an electro-mechanical energy conversion element is bonded to the other surface of the elastic member 1, i.e., the surface without the grooves, as shown in FIG. 17, and a pattern electrode 2-1 shown in FIG. 17 is formed in the piezoelectric element 2 by evaporation or by printing.
The pattern electrode 2-1 is equally divided in a number equal to four times an order of a vibration mode excited in the ring portion of the vibration member, and alternating voltages of nearly sinusoidal shape with time phases successively shifted at intervals of 90° are applied to the respective electrodes. When the alternating voltages are applied at a frequency near the natural frequency of the excited vibration mode, the elastic member resonates because of the bending moment exerted on the elastic member by expansion and contraction of the piezoelectric element 2, so that vibrations are generated corresponding to the respective alternating voltages different at intervals of 90°. These vibrations are combined to form a travelling wave. Since the exciting portions of the piezoelectric element are equally distributed in the ring-shaped vibration portion as described, the amplitude of vibration is uniformized and highly accurate driving can be maintained over a long period of time.
A technique for reducing noise, so called “squeaks,” is disclosed in Japanese Patent Application Laid-Open No. 2-214477. In the case where a vibration mode different from the mode used for driving is generated as a self-excited vibration by an exciting force appearing at a contact part, dynamic stiffness is made nonuniform at positions corresponding to integral multiples of half of the wavelength of the mode, whereby natural frequencies of vibrations in the same mode are made different from each other, so as to impede production of a travelling wave. Since the exciting force due to the contact between the movable member and the vibration member acts on the contact part, squeaks can be made in some cases, but the above means can make the squeaks less prone to be generated. Grooves 4-1 in FIG. 18 are deeper than the other grooves 4 and are located at sixteen positions throughout the circumference. This makes a difference between natural frequencies of two vibrations in the eighth-order mode with nodes at the sixteen positions and makes the self-excited vibration less prone to appear as a travelling wave.
There were, however, cases where it was difficult to maintain the performance over a long period of time even by the uniformization of vibration or by the decrease of the self-excited vibration of the other mode as in the conventional configuration.
FIG. 19 provides other natural modes (the fifth-order and sixth-order torsion modes and the second-order and third-order in-plane bending modes) near the driving mode of the conventional elastic member (the ninth-order out-of-plane bending mode), and natural frequencies thereof This vibration member utilizes, for driving, the out-of-plane ninth-order mode in which the ring portion undergoes deflection in the axial direction, as in a diagram of deformation of the elastic member shown in FIG. 20. In this example, there exist the out-of-plane eighth-order mode lower in the order and the out-of-plane tenth-order mode higher in the order than the out-of-plane ninth-order mode used for driving, in the lower frequency and upper frequency regions than the frequency of the out-of-plane ninth-order mode.
Besides the out-of-plane modes, as shown in FIG. 21, there exist torsion modes in which the ring portion is alternately twisted with respect to the axis near the center of the cross section of the ring portion, and in-plane modes in which the ring portion undergoes bending vibration in the plane normal to the symmetry axis of the ring.
Since the exciting forces by the electrode pattern of the piezoelectric element shown in FIG. 17 are exerted at excitation points of nine positions equal in phase, which are arranged equally on the circumference, the exciting forces except for the ninth-order mode are canceled out and do not appear accordingly. For this reason, there are no exciting forces of the other modes near the driving frequency.
A curve “A” in FIG. 22 is a frequency response curve of vibration displacement to the frequency of the driving voltages applied to the piezoelectric element in a non-contact state of the vibration member with the movable member. Since there are no exciting forces of the other modes, no response appears in the other modes.
A curve “B” in FIG. 22 indicates a frequency response curve in the case where the frictional portion of the elastic member is excited at nine points arranged at equal intervals. In this case, similar to the case of the curve “A” the response curve includes only a response in the ninth-order mode.
On the other hand, the exciting forces on the vibration member are resultant forces of two kinds, the exciting forces from the piezoelectric element and the exciting forces from the movable member in press contact, and thus the vibration member must experience the ninth-order excitation from the piezoelectric element and the even exciting forces in the nine regions in contact with the movable member in the driving state of the movable member. Therefore, no other mode is forcedly excited in the driving state.
However, a response is different if the frictional member is uneven. Supposing the frictional member has a projection at only one point, the pressure of contact with the movable member is concentrated at one point of the projection, and the pressure is lowered at the other contact portions. The vibration member undergoes excitation at the driving frequency at the projecting point every time the travelling wave of the driving vibration passes the projecting point.
A curve “C” in FIG. 22 represents a frequency response curve in the case where the frictional portion of the vibration member is excited at one point. It is seen therefrom that there appear responses in the other modes, different from the case of equal excitation at nine points.
FIGS. 23A and 23B show responses of an out-of-plane bending mode and torsion modes as separate response curves. FIG. 23A shows the response curves in the case of a small amplitude of vibration (low rotational speed) and FIG. 23B shows the response curves in the case of a large amplitude of vibration (high rotational speed). The response curve of the out-of-plane bending ninth-order mode demonstrates such nonlinearity that the resonant frequency gradually decreases with increase of amplitude, because of change of the contact state caused by the increase of amplitude.
Since the out-of-plane bending ninth-order mode is used as the driving mode herein, excitation is induced in the driving frequency band as illustrated. At this time, the torsion fifth-order mode close to the driving mode exhibits a large response in the frequency band used for the driving. This raises a concern that in the driving state with the frictional portion being-uneven, the forced excitation at the driving frequency produces the torsion fifth-order vibration and the torsion fifth-order vibration is superimposed on the out-of-plane ninth-order vibration of the driving mode.
An amplitude distribution of the vibration member was actually measured in the state of the frictional portion being uneven and the amplitude distribution obtained was that as shown in FIG. 24. This distribution has amplitude maxima at fourteen positions, because the torsion fifth-order mode is superimposed at the same frequency on the ninth-order mode.
This is because the unnecessary other mode appears in response to the driving frequency and the unwanted vibration also appears as a vibration at the driving frequency. For this reason, it never makes noise, like squeaks.
However, the vibration amplitude of the composite vibration becomes uneven, which promotes partial abrasion of the frictional portion and causes localized abrasion. With advance of localized abrasion at the fourteen positions, the clearance will expand relative to the vibration member driving in the ninth-order mode and the fifth-order mode becomes more likely to arise. This will result in further promoting the localized abrasion and end up in failure in maintaining stable contact and degrading output characteristics.
The unevenness of the frictional portion is made by flaws during production, temporary deposition of abrasion powder in the driving state, and dropping of a filler, and the exciting forces from the movable member due to the unevenness made thereby can be the exciting forces to excite the other mode at the driving frequency.